This invention relates to power amplifier circuits and in particular, to such circuits having a differential amplifier at an input stage and a complementary symmetry push-pull amplifier at an output stage.
As a power amplifier for audio-signals, a circuit has been well known wherein one or two differential amplifiers are used at an input stage, the output from the input stage being applied to complementary symmetry circuits for power amplification.
Referring to FIG. 1, an example of known power amplifiers comprises a differential amplifier DA.sub.1, including a pair of FET transistors Tr.sub.1 and Tr.sub.2 to which an input signal and a fed-back signal are applied, respectively. Two opposite phase output signals from the differential amplifier DA.sub.1 are applied to two inputs of another differential amplifier DA.sub.2 including a pair of NPN transistors Tr.sub.3 and Tr.sub.4.
An output of the differential amplifier DA.sub.2 is applied to a complementary symmetry circuit CC.sub.1 including a pair of transitors Tr.sub.5 and Tr.sub.6 of opposite conductive types. In the shown circuit, a collector output of the NPN transistor Tr.sub.4 is applied to a base electrode of the PNP transistor Tr.sub.5 of the complementary symmetry circuit CC.sub.1.
The other NPN transistor TR.sub.6 of the circuit CC.sub.1 is base-biassed at a constant voltage defined by a voltage divider constituted by resistors Ra and Rb. Thus, the transistor Tr.sub.6 serves for a constant current means.
The output of the complementary symmetry circuit CC.sub.1 is applied to two inputs of a complementary symmetry push-pull circuit CC.sub.2 including a pair of transistors Tr.sub.7 and Tr.sub.8 of opposite conductive types, to which another complementary symmetry push-pull circuit CC.sub.3 including a pair of transistors Tr.sub.9 and Tr.sub.10 is connected.
A power amplified output signal is obtained from the complementary symmetry circuit CC.sub.3 at an output terminal OUT. A portion of the output signal is fed-back to the differential amplifier DA.sub.1, as shown.
Diodes BD.sub.1 are a biassing circuit from transistors Tr.sub.7 -Tr.sub.10.
The differential amplifiers DA.sub.1 and DA.sub.2, the voltage divider, and the complementary symmetry circuits CC.sub.1 -CC.sub.3 are connected in parallel with one another between a positive and a negative power lines which are connected to a positive and a negative power sources +Vcc and -Vcc.
In the known power and amplifier circuit, only one output from the differentail amplifier DA.sub.2 is used to drive the following complementary symmetry circuit CC.sub.1 and one of the transistors, or Tr.sub.6, of the complementary symmetry circuit CC.sub.1 does not operate to amplify any signal.
On the other hand, a power amplifier circuit has been also used wherein both of two outputs of a differential amplifier are used as input signals to a following complementary symmetry circuit. An example of such power amplifier circuits is shown in FIG. 2.
Referring to FIG. 2, similar parts are respresented by same reference characters as in FIG. 1. The shown circuit is similar as the amplifier circuit in FIG. 1 except that not only one of the outputs of the differential amplifier DA.sub.2 but also the other is coupled with the complementary symmetry circuits CC.sub.1. Namely, one of the opposite phase outputs of the differential amplifier DA.sub.2 is directly applied to the transistor Tr.sub.5 of the complementary symmetry circuit CC.sub.1, and the other is coupled with the transitor Tr.sub.6 through a PNP transistor Tr.sub.11 which is so provided to form a differential amplifier, together with the transistor Tr.sub.5 of the complementary symmetry circuit CC.sub.1. The base of the transistor Tr.sub.11 is connected to the collector of the transistor Tr.sub.3. The collector of the transistor Tr.sub.11 is connected to the base of the transistor Tr.sub.6 of the complementary symmetry circuit CC.sub.1, and is connected to the negative power line through a diode D.sub.1 and a resistor Rc.
In the circuit arrangement, the transistor Tr.sub.6 and the diode D.sub.1 form a current mirror circuit so that the current value i.sub.1 on the collector circuit of the transistor Tr.sub.6 may consist with the current i.sub.2 on the collector circuit of the transistor Tr.sub.11. Thus, the opposite phase output signals of the differential amplifier DA.sub.2 are applied to the complementary symmetry circuit CC.sub.1, after being converted into in-phase signals. Therefore, the complementary symmetry circuit CC.sub.1 operates as a push-pull amplifier.
The use of opposite phase output signals from the differential amplifier DA.sub.2 to operate the complementary symmetry circuit CC.sub.1 as a push-pull amplifier is known to provide various advantages, for example, that the distortion of the output is reduced, that any variation and noise of a used power source do not present on the output and that the output drift depending on the ambient temperature is reduced.
However, the circuit arrangement as shown in FIG. 2 has a disadvantage in economical view point.
The transistor Tr.sub.11 must be designated to permit flowing on its collector circuits a DC current of a same value as the transistor Tr.sub.5, because both of those transistors form a differential amplifier and because the current mirror circuit is provided in the collector circuits of the both transistors Tr.sub.5 and Tr.sub.11.
Usually, DC current of about 10mA is required as the collector current i.sub.1 of the transistor Tr.sub.5. If the voltage of the power source Vcc is 50V, a transistor of a power capacity of 0.5W or more must be used for the transistor Tr.sub.5.
Therefore, the transistor Tr.sub.11 must be of a transistor having a power capacity of 1W or more, because the transistor Tr.sub.11 is connected between a positive and a negative power sources +Vcc and -Vcc so that a voltage of about 100V (= 2 .times. Vcc) is applied to the transistor Tr.sub.11, and because the collector current i.sub.2 of the transistor Tr.sub.11 is equal to the current i.sub.1 (= 10mA).
On the other hand, transistor Tr.sub.11 must have a same property as the transistor Tr.sub.5. The transistor Tr.sub.5 must, also, have a same property as the transistor Tr.sub.6 except the conductive type.
Accordingly, the use of the transistor Tr.sub.11 requires the use of transistors of a larger power capacity than the necessary power capacity as transistors Tr.sub.5 and Tr.sub.6.
Furthermore, the complementary symmetry circuit CC.sub.1 operates only as an A-class amplifier because the transistor Tr.sub.5 forms a differential amplifier incorporating with the transistor Tr.sub.11.